The present invention pertains to the field of liquid crystal displays.
Liquid crystal molecules are elongated and a vector pointing along the elongated axis is called an orientational director. Liquid crystal displays exhibiting bistable equilibrium orientational direction configurations are useful in purely field effect storage displays, for example see an article entitled "Liquid-Crystal Orientational Bistability and Nematic Storage Effects", Applied Physics Letters, Vol. 36, No. 7, Apr. 1, 1980, by G. D. Boyd, Julian Cheng and P. D. T. Ngo, pp. 556-558. The orientational director configurations are bistable in the sense that the pattern of alignment in a first configuration is not altered until energy is applied to a region of the display called a cell, e.g., by an applied electric field. The applied electric field causes a first configuration to change to a second configuration, which second configuration remains substantially unaltered when the applied electric field is removed. The bistable liquid crystal configurations are produced by specific treatments of the substrate surfaces of liquid crystal displays, which treatments cause characteristic orientational director alignment thereat.
A display cell, known in FIG. 1 of the above-cited reference and called the single-tilt (ST) geometry, comprises two substrates with a liquid crystal material disposed therebetween. Each substrate surface is treated so that the liquid crystal orientational directors have a uniformly tilted boundary alignment at the substrate surfaces, which alignment is symmetrical about the midplane of the cell. The equilibrium states of the display cell are primarily horizontal or vertical in orientational director alignment in the volume of the cell and are thus differentiable optically, e.g. by the incorporation of pleochroic dyes or in many cases by the use of crossed polarizers.
A second display cell, shown in FIG. 2 of the above-cited reference and called the alternating-tilt (AT) geometry, comprises an array of ST cells with neighboring cells having oppositely tilted surface alignment. The AT geometry possesses equilibrium states and properties analogous to those of the ST geometry.
The bistable states of the liquid crystal cell are topologically inequivalent and switching therebetween in either the ST or the AT geometry requires the detachment and motion of disclinations in the cell, which motion is a transit-time limited process. (A disclination is a discontinuity in the orientation of orientational directors. The discontinuity may be located at one point or on a line.) Each state of a cell is stable only if the disclinations that define the cell boundaries are pinned to one of the cell boundaries. Otherwise, the disclinations will drift from the cell boundaries into the display cell and cause one state to switch into the other. Thus, it is necessary to provide appropriate substrate surface boundary conditions with which to surround the display cells, which substrate surface boundary conditions will pin disclinations and thereby provide stabilization of the states of the display cells. These substrate surface boundary conditions will also serve to separate display cells.